WHAT I HAVE:   subject   val (char) Date        visit 01          IL01         June 1       1 01          IL02         June 1       1 01          .               July 1         2 01          IL02          July 1        2 01          IL03           July 1      2 01          IL04          July 1       2 01          IL01          Aug 1        3 01          .                Aug 1        3 01          IL03         Aug 1         3 01          IL04         Aug 1         3   what I want:   subject       VAL           Date           visit     Flag  01              IL01           July 1          2       error 01              IL02             Aug 1         3      error   I am trying to find a way to highlight that IL01 which was present on June 1 at visit 1 is not present on July 1 at visit 2 and then IL02 was not present in Aug 1 at visit 3 but present in July 1 at visit 2. Basically comparing the current visit data to the prior visit data for the absence of the value. meaning that whatever is present at the prior visit in VAL must be present at the next visit and so forth. hope this makes sense, I do not want the 1st visit to flag since there is nothing prior to June 1.     any help appreciated.     ... View more